For example, there is provided a vehicle headlamp where an incandescent lamp (incandescent bulb) or a halogen lamp (halogen bulb) is used as a light source, a vehicle headlamp where a discharge lamp (discharge bulb) is used as a light source, and a vehicle headlamp where a light emitting diode (LED) is used as a light source.
Since light intensity and brightness of a discharge lamp is larger than those of an incandescent lamp, a halogen lamp, and light emitting diode, it may be possible to obtain an advantage of achieving a headlamp having a large light intensity in the case of the vehicle headlamp where a discharge lamp is used as a light source.
A discharge lamp has a double-tube structure where a light emitting tube in which a pair of electrodes is held and gases such as noble gases or metal halides are sealed is disposed in an outer tube. The light emitting tube includes a light emitting portion where discharge occurs, and a pair of pinch seal portions that is provided on the opposite sides of the light emitting portion and is made of a material such as quartz glass. The light emitting portion is blocked from the outside by the pinch seal portions. The light emitting portion is a portion where arcs are generated during the occurrence of discharge, and has a diameter larger than the diameter of the pinch seal portion.
One end portions of the pair of electrodes protrude into the light emitting portion, and the other end portions thereof are sealed in the pinch seal portions, respectively. The other end portions of the pair of electrodes are welded to molybdenum foils sealed in the pair of pinch seal portions, respectively.
In the discharge lamp, a predetermined starting voltage is applied to the pair of electrodes held in the light emitting tube, so that discharge occurs in the light emitting portion of the light emitting tube. Accordingly, the discharge lamp starts to be turned on.
The discharge lamp is pinch-sealed by molybdenum foils in order to block completely the light emitting tube from the outside.
The molybdenum foil is formed of a very thin foil in order to minimize stress generated due to the difference between coefficients of thermal expansion of the quartz glass of the pinch seal portion and the molybdenum foil which is caused by temperature change occurring due to the repeated turning on/off of the discharge lamp.
However, if the discharge lamp is repeatedly turned on/off several tens of thousands times, the molybdenum foil and quartz glass are gradually separated due to stress (cracks are generated). Eventually, non-lighting due to the leakage occurs (the life of the discharge lamp comes to an end).
The following two methods are generally used to prevent the non-lighting caused by the molybdenum foil and to lengthen the life of the molybdenum foil.
(1) A method of increasing the adhesion between the molybdenum foil and quartz glass.
(2) A method of making the molybdenum foil distant from the light emitting portion, that is, a method of increasing the length of the electrode.
However, in the method (1) of increasing the adhesion between the molybdenum foil and quartz glass, the adhesion between the electrode and quartz glass is also increased. For this reason, the difference between the coefficients of thermal expansion of the electrode of the pinch seal portion is caused by temperature change occurring due to the repeated turning on/off of the discharge lamp. As a result, stress is generated and cracks are generated in the pinch seal portions around the electrode, so that the non-lighting due to the leakage occurs.
In recent years, a discharge lamp where mercury is not sealed in a light emitting tube has been widely used in consideration of the environment. However, since mercury is not sealed in this discharge lamp, a tube voltage is not increased. As a result, tube current is increased, and the outer diameter of the electrode should be increased in order to cope with the increase of tube current. For this reason, stress, which is generated between the electrode and the pinch seal portion, is further increased, so that there is a high possibility that cracks are generated in the pinch seal portion, which leads to occurrence of the non-lighting due to the leakage.
In contrast, if the adhesion between the molybdenum foil and quartz glass is decreased, the life of the molybdenum foil is reduced and a gap is formed between the electrode and the quartz glass. Accordingly, there is also a possibility that non-lighting occurs due to the leakage from the gap.
Further, in the method (2) of making the molybdenum foil distant from the light emitting portion, that is, the method of increasing the length of the electrode, the length of the electrode adhering to the quartz glass is increased. Accordingly, stress, which is generated between the electrode and the pinch seal portion, is increased, so that cracks are generated in the pinch seal portion around the electrode, which leads to occurrence of the non-lighting due to the leakage.
In order to prevent the generation of a leak from the electrode that is caused by the cracks, the following discharge lamp has been proposed as a discharge lamp in the related art (for example, see Patent Document 1). In this discharge lamp, very small gaps are formed between an electrode and an inner surface of a pinch seal portion by a metal wire wound on the electrode in the shape of a coil, and a sealing property is secured with appropriate adhesion between the electrode and quartz glass. Accordingly, non-lighting due to the leakage is prevented and the life of the discharge lamp is lengthened.
An example of such configuration is disclosed in JP-A-2008-181844.
However, the discharge lamp disclosed in the publication, JP-A-2008-0181844, metal halides (iodide) sealed in the light emitting portion permeate into the gaps, which are formed between the electrode and the inner surface of the pinch seal portion, in the form of a gas due to high pressure at the time of turning-on/off of the discharge lamp, and reach the molybdenum foil. If the iodide reaches the molybdenum foil, the iodide is solidified at the time of the turning-off of the discharge lamp. For this reason, stress, which makes the molybdenum foil separate from the pinch seal portion, is generated, and the leak of molybdenum foil from a sealed portion (foil leak) occurs due to the decrease in the adhesion between the pinch seal portion and the molybdenum foil. As a result, non-lighting occurs and the life of the discharge lamp is decreased.